1. Field of the Invention
This invention relates to a supercharging pressure control system for supercharged internal combustion engines.
2. Prior Art
Conventionally, there has been proposed, e.g. by Japanese Provisional Utility Model Publication (Kokai) No. 61-17141, a supercharging pressure control system for a supercharged internal combustion engine which has a supercharger arranged in the intake system of the engine, the supercharger being connected to the crankshaft of the engine, a bypass passage bypassing the supercharger, and bypass passage control means for opening and closing the bypass passage depending on operating conditions of the engine. The supercharging control system comprises sensor means for detecting the temperature of the engine at one or more points, and bypass passage-closing means for driving the bypass passage control means such that the bypass passage is fully closed when the sensor means detects that the engine is low in temperature. According to this prior art, when the sensor means detects a low temperature cold condition of the engine at the start of the engine, the bypass passage is fully closed to effect supercharging to the maximum degree, whereby warming-up of the engine is accelerated.
However, in the prior art system, upon starting of the engine under a cold condition, the bypass passage is fully closed to effect supercharging to the maximum degree, which results in wasteful consumption of a large amount of fuel to increase the fuel consumption. Further, a catalytic converter arranged in the exhaust system exhibits a low purifying efficiency (reactivity) before the temperature of exhaust gases flowing thereinto reaches a predetermined value, involving a problem of emission of large amounts of unpurified exhaust gases, which is caused by supercharging carried out at the start of the engine under a cold condition, resulting in degraded exhaust emission characteristics.
On the other hand, when the engine continues to be in a high speed or high load operating condition for a long time, such as when the vehicle is running at a high speed or going up a slope, the temperature of the exhaust gases may rise to such a degree as causes deterioration of the catalytic converter arranged in the exhaust system.
To protect the catalytic converter from such a deterioration of the catalyst converter, an air-fuel ratio control system has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 60-3421, in which the air-fuel ratio of an air-fuel mixture supplied to the engine is feedback-controlled to a stoichiometric ratio or the vicinity thereof, in response to the actual air-fuel ratio detected based on a signal from an O.sub.2 sensor, the system being adapted to stop the feedback control and hold the air-fuel ratio at a value richer than the stoichiometric value in response to a signal supplied from an exhaust gas temperature sensor mounted in the catalytic converter when the sensor signal shows that the temperature of exhaust gases is higher than a predetermined value, to thereby lower the temperature of exhaust gases to protect the catalytic converter from being deteriorated.
However, in this prior art system, while the air-fuel ratio is held at an enriched value after the temperature of exhaust gases becomes higher than the predetermined value, ingredients of exhaust gases, such as CO, HC, and NOx, cannot be properly purified, resulting in degraded exhaust emission characteristics, as well as in wasteful consumption of fuel or increased fuel consumption.